All right, it’s not completely flat: the new rate of 32.5 per cent will apply to earnings between $41,000 (£23,000) and $200,000 (£111,000). And it won’t be fully in place until 2024. Still, Australia is the first wealthy Western country even to aspire to a single rate. Hitherto, flat taxes have generally been implemented on small islands and in former Soviet territories.

Why haven’t they caught on more widely? After all, we have enough evidence to know that, other things being equal, evening out tax rates boosts...